Chondrichthyes │ └─Neoselachii ├─Galeomorphii │ ├─Orectolobiformes │ └─┬─Carcharhiniformes │ └─Lamniformes └─Squalea ├─Squalomorpha └─Hypnosqualea ├─Rajiformes └─Myliobatoidei
The Neoselachii are the living sharks and rays. In fact, one current cladistic definition is precisely "the last common ancestor of all living sharks and rays and all of its descendants." This is a cumbersome and ultimately circular definition, so we will instead think of it as Raja + "Jaws" (Carcharodon). This crown group is, in turn, made up of two major clades and an extinct orphan order.
The extinct order is the Synechodontiformes. For reasons of convenience and tradition, this group is treated in the previous section. The two major living taxa are the Galeomorphii and the Squalea. The Galeomorphii are, roughly speaking, the "typical" sharks with 5 gill slits. When we think "shark" we typically visualize either a galeomorph or a lawyer. Fortunately, these two similar forms can easily be distinguished by their billing rates.
The Squalea are the sharks with 6 gill slits (Squalomorpha) plus the Batomorphii. The latter are, morphologically, a very odd group. They include things like the electric rays (Torpediniformes) and the swordfishes (Pristidae). Below, we discuss in somewhat more detail the fundamental divide between the galeomorphs and squaleans. ATW020803.
At some point in the early 1990's the Galeomorphii seem to have undergone a sex-change operation. In the high and far-off times before about 1994, the taxon was referred to as Galeomorpha; but, after that date, as Galeomorphii. Obviously bucking some very strong historical and political trends, the name was somehow transmuted from the neuter plural to the masculine plural. This makes a sort of sense. The underlying genus is Galeus, one of the catshark genera. Galeus is a garden-variety Latin second declension masculine name -- presumably Latinized from galeos, a Greek word for "shark." There is no rule from the ICZN which requires the "morph" form of the word to agree in gender with the underlying genus name, and arguably that isn't even grammatical. However, there seems no harm in the practice. Morphé (= form) is a Greek word, so the gender of its Latinized version may perhaps be regarded as arbitrary. What is harder to understand is the double 'i'. Just plain Galeomorphi should be sufficient. The double 'i' suggests that the root might be Morpheus, the personification of sleep or dreaming. Thus, instead of Galeomorphi, the "shark-like forms," we have Galeomorphii, perhaps the "dream sharks." (Image: Under the Sea © David Thomas and reproduced by permission.) This has a truly memorable hallucinogenic quality, but is probably not the intended sense.
But enough linguistic pedantry. On with the scientific pedantry! The Galeomorphii are in fact the sharks of nightmare, including Carcharodon, the Great White, and its even larger ancestors. Not all galeomorphs are top predators, of course. In fact, the most successful and speciose group are not the lamniforms (mackerel sharks), to which Carcharodon belongs, but the carcharhiniforms, or ground sharks, many of which are large, but not notably aggressive.
The galeomorphs appear to have diverged from the Squalea in the Early Jurassic. However, for all the fossil record tells us, the group was restricted to a few heterodontiforms until the Cretaceous, when almost all of the modern families appear, more or less at the same time. This sudden appearance may be an artifact of the very limited fossil record, but it is worth noting that the record of the Squalea is quite different. The squalean sharks, rays and skates show a gradual, possibly continuous pattern of diversification from the mid-Jurassic to the Recent. Thus, the apparent explosive radiation of galeomorphs in the mid-Cretaceous may be an accurate reflection of their actual evolutionary development.
Although the origins of modern chondrichthyans, the Neoselachii, are not well understood, the early diversification into galeomorphs and squaleans appears to be quite well characterized. When Shirai 1996) and deCarvalho (1996) published their back-to-back analyses of neoselachian evolution, they not only produced almost identical trees, but came up with very similar lists of identifying synapomorphies. This is an almost unheard of level of agreement for a basal radiation in cladistics, and it allows us to develop some fairly serious ideas about the reasons for the divergence. That is, we may know enough about what happened to sharks in the Cretaceous that we can begin to ask why.
The general understanding (i.e., the understanding of Pough, et al (1999)) is that neoselachians possessed certain key adaptations including a partially calcified vertebral column analogous to the vertebral column of tetrapods, although derived quite independently. In addition, neoselachians consolidated the pectoral girdle, which not only created the potential for faster swimming, but could serve as an anchor for some of the jaw musculature. The upper jaw was loosened from its tight connections with the massive braincase and was increasingly braced against the hyomandibula. This development allowed the jaws to protrude, in a manner analogous to (but again independently derived from) teleost fish, and freed the braincase from part of its structural role. As a result, the neoselachians developed additional or more refined sensory structures and larger brain capacity. In fact, many galeomorphs may rival at least some birds and mammals in braininess.
Against this background, the fundamental split between galeomorphs and squaleans may have developed because one lineage, the galeomorphs, continued and "improved" on the traits that had given neoselachians their niche in the first instance, while the squaleans developed specializations in the "reverse" direction. Thus, the galeomorphs further consolidated the pectoral girdle, evolved a variety of more fully streamlined, fusiform bodies, and added more bone to the vertebrae. The articulation between the upper jaw and the braincase moved back on the skull. The posterior end of the gill skeleton also consolidated, as evidenced by, for example, the development of the pharyngobranchial "pick-axe". The general result was a body with even greater central strength, and jaws of great strength and flexibility: faster, stronger fish with a very broad range of prey.
The squaleans developed jaw mobility to an even greater degree. In doing so, they may have crested an evolutionary peak from which the best road led into an entirely different morphospace than the traditional home of the Neoselachii. At some point, jaw flexibility begins to be expensive in terms of jaw strength. The mobility of the jaw requires a more stable platform from which to launch it. In any case, given an enlarged reaction space, but a weaker jaw, it may be more advantageous to cruise slowly and efficiently, or wait in ambush, rather than sprint around trying to engulf things. Thus, instead of consolidating power on the long axis of the body, squaleans tended to develop lateral stability with broad basal cartilages in the pectoral fins. Instead of consolidating the gill skeleton, they tended to develop increased respiratory flexibility by increasing the number of gills. Ultimately, new squalean forms broadly expanded the pectoral fins to dominate the entire body form: the skates and rays. Interestingly, the galeomorphs and squaleans tend to be ecologically partitioned as well. The galeomorphs are typically found as free-swimming species in warm, shallow waters with relatively higher prey densities. The squaleans tend to be benthic or inhabit colder and deeper waters.
Is this coincidence, teleological fate or some long-term evolutionary trajectory? Perhaps not. At least there may be an explanation which does not require the invocation of external forces acting over millions of years. Perhaps the principal evolutionary force acting on the squaleans was the galeomorphs, and vice-versa. As rather non-selective top predators, it would not be surprising if the primary selective pressures on neoselachians had been other neoselachians. Prey adaptations would generally be of little consequence, since one prey animal could always be substituted for another. Short of major ecological collapse, a top predator's chief headache is other top predators. So, if the original galeomorphs were only very slightly faster, stronger, or brighter than their squalean brethren, nothing much might happen for quite a long time. The difference might well be made up by the squaleans' lateral stability, more accurate bite, and so on. However, the process may be viewed as a positive feedback mechanism. Over a long enough period of time, small advantages in pelagic, non-specific feeding by galeomorphs would be more probably offset by a counter-drift by squaleans toward specialized prey, benthic or low prey density regions, and physical adaptations stressing the opposite strengths from those of the competition. After all if one is a squalean with pectoral fin basals, and the home turf gets crowded with fast, nasty torpedo-shaped galeomorphs, what favorable mutation in my offspring is more likely to occur: (a) bigger pectoral fins which allow the kids to skim effortlessly along the bottom picking up things the galeomorphs can't see or get to or (b) some miraculous centralization which will allow them to out-swim a Great White? So chance alone increasingly favors opposite specializations once an initial, random divergence occurs.
Whether this might be true outside the unusual world of top predators is too much speculation even for this forum. But it seems a phenomenon worth watching for. ATW010203, revised 021018.